Proteases are involved in a broad range of biological pathways. In particular, serine proteases such as kallikrein, plasmin, elastase, urokinase plasminogen activator, thrombin, human lipoprotein-associated coagulation inhibitor, and coagulation factors such as factors VIIa, IXa, Xa, XIa, and XIIa have been implicated in pathways affecting blood flow, e.g., general and focal ischemia, tumor invasion, fibrinolysis, perioperative blood loss, and inflammation. Inhibitors of specific serine proteases, therefore, have received attention as potential drug targets for various ischemic maladies.
One such inhibitor, aprotinin (also called bovine pancreatic trypsin inhibitor or BPTI), obtained from bovine lung, has been approved in the United States for prophylactic use in reducing perioperative blood loss and the need for transfusion in patients undergoing cardiopulmonary bypass (CPB), e.g., in the course of a coronary artery bypass grafting procedure. Aprotinin is commercially available under the trade name TRASYLOL® (Bayer Corporation Pharmaceutical Division, West Haven, Conn.) and was previously approved for use to treat pancreatitis. The effectiveness of aprotinin is associated with its relatively non-specific abilities to inhibit a variety of serine proteases, including plasma kallikrein and plasmin. These proteases are important in a number of pathways of the contact activation system (CAS).
CAS is initially activated when whole blood contacts the surface of foreign substrates (e.g., kaolin, glass, dextran sulfate, or damaged bone surfaces). Kallikrein, a serine protease, is a plasma enzyme that initiates the CAS cascade leading to activation of neutrophils, plasmin, coagulation, and various kinins. Kallikrein is secreted as a zymogen (pre-kallikrein) that circulates as an inactive molecule until activated by a proteolytic event early in the contact activation cascade. Clearly, specific inhibition of kallikrein would be a very attractive approach to control blood loss associated with CPB and the onset of systemic inflammatory response (SIR) as would be encountered during, for example, various invasive surgical procedures:
Despite being the only licensed compound for preventing perioperative blood loss in CPB for coronary artery bypass grafting (CABG) procedures, aprotinin is not as widely used as would be expected. There are serious concerns regarding the use of this bovine polypeptide in patients who require CPB, and in particular the use of this compound in CABG procedures. Aprotinin is not specific for kallikrein, but interacts with additional enzymes (e.g., plasmin) in multiple pathways. Thus, the mechanism of action of aprotinin is largely speculative, and the lack of precise understanding of what is affected during aprotinin treatment produces the risk of complications during treatment. One frequently cited complication is uncontrolled thrombosis, due to aprotinin's actions upon the fibrinolytic pathway. There is concern not only over such hyperacute events as major vessel thrombosis in the perioperative period, but also over graft patency after the CABG procedure. Furthermore, as a naturally occurring protein obtained from bovine lung, administration of aprotinin in humans can elicit severe hypersensitivity or anaphylactic or anaphylactoid reactions after the first and, more often, after repeat administration to patients. This is particularly of concern in the large number of patients who have repeat CABG procedures. In addition, there is an increasing public concern regarding use of material derived from bovine sources as a potential vector for the transmission of bovine spongiform encephalopathy to humans.
These concerns make clear that a need remains for more effective and more specific means and methods for preventing or reducing perioperative blood loss and the onset of SIR in a patient subjected to surgery resulting in activation of the CAS, such as CABG procedures in patients of CPB, or hip replacement.